Search Results (9)

In the process of collecting operational data for the performance analysis of water-cooled centrifugal chillers, missing values are inevitable due to various factors such as sensor errors, data transmission failures, and failure of the measurement system. When a substantial amount of missing data is present, the reliability of data analysis decreases, leading to potential distortions in the results. To address this issue, it is necessary to either minimize missing occurrences by utilizing high-precision measurement equipment or apply reliable imputation techniques to compensate for missing values. This study focuses on two water-cooled turbo chillers installed in Tower A, Seoul, collecting a total of 118,464 data points over 3 years and 4 months. The dataset includes chilled water inlet and outlet temperatures ($T_1$ and $T_2$) and flow rate (${\dot{V}}_1$) and cooling water inlet and outlet temperatures ($T_3$ and $T_4$) and flow rate (${\dot{V}}_3$), as well as chiller power consumption (${\dot{W}}_c$). To evaluate the performance of various imputation techniques, we introduced missing values at a rate of 10–30% under the assumption of a missing-at-random (MAR) mechanism. Seven different imputation methods—mean, median, linear interpolation, multiple imputation, simple random imputation, k-nearest neighbors (KNN), and the dynamically clustered KNN (DC-KNN)—were applied, and their imputation performance was validated using MAPE and CVRMSE metrics. The DC-KNN method, developed in this study, improves upon conventional KNN imputation by integrating clustering and dynamic weighting mechanisms. The results indicate that DC-KNN achieved the highest predictive performance, with MAPE ranging from 9.74% to 10.30% and CVRMSE ranging from 12.19% to 13.43%. Finally, for the missing data recorded in July 2023, we applied the most effective DC-KNN method to generate imputed values that reflect the characteristics of the studied site, which employs an ice thermal energy storage system. Full article

In this study, a regression model was developed using the thermo-regulated residual refinement regression model (TRRM) analysis method based on three years and four months of in situ data collected from two water-cooled centrifugal chillers installed in A Tower, Seoul, South Korea. The primary objective of this study was to predict the coefficient of performance (COP) of water-cooled chillers under various operating conditions using only the chilled water outlet temperature ($T_2$) and the cooling water inlet temperature ($T_3$). The secondary objective was to estimate the COP under standard temperature conditions, which is essential for the absolute performance evaluation of chillers. The collected dataset was refined through thermodynamic preprocessing, including the removal of missing values and outliers, to ensure high data reliability. Based on this refined dataset, regression analyses were conducted separately for four cases: daytime (09:00–21:00) and nighttime (21:00–09:00) operations of chiller #1 and chiller #2, resulting in the derivation of four final regression equations. The reliability of the final dataset was further validated by applying other regression models, including simple linear (SL), bi-quadratic (BQ), and multivariate polynomial (MP) regression. The performance of each model was evaluated by calculating the coefficient of determination ($R^2$), coefficient of variation of root mean square error (CVRMSE), and the p-values of each coefficient. Additionally, the predicted COP values under the design and standard temperature conditions were compared with the measured COP values to assess the accuracy of the model. Error rates were also analyzed under scenarios where $T_2$ and $T_3$ were each varied by ±1 °C. To ensure robust validation, a final comparison was performed between the predicted and measured COP values. The results demonstrated that the TRRM exhibited high reliability and predictive accuracy, with most regression equations achieving $R^2$ values exceeding 90%, CVRMSE below 5%, and p-values below 0.05. Furthermore, the predicted COP values closely matched the actual measured COP values, further confirming the reliability of the regression model and equations. This study provides a practical method for estimating the COP of water-cooled chillers under standard temperature conditions or other operational conditions using only $T_2$ and $T_3$. This methodology can be utilized for objective performance assessments of chillers at various sites, supporting the development of effective maintenance strategies and performance optimization plans. Full article

This paper presents the development and use of benchmarking grey-box models for the detection and diagnosis of multiple-dependent faults (MDFDD) of a water-cooled centrifugal chiller. Models are developed using data recorded by a Building Automation System (BAS) from a central cooling plant of an institutional building. The forward residual-based fault detection model identifies a fault symptom, when the difference between the measured value of target variable and benchmarking value exceeds the corresponding threshold. For the fault diagnosis, most publications start from a known single fault and establish the impact on following variables in the system. This paper presents a rule-based backward approach. The proposed method identifies if (i) the fault symptom is correct (i.e., a variable has abnormal values), or (ii) the fault symptom is incorrect (i.e., the symptom of target variable is caused by impacts generated by other faulty variables due to the dependency between variables), or (iii) both target and regressor variables are abnormal. For testing the proposed MDFDD model, some artificial faults are inserted into the measurement data file, and results are discussed about the method potential for the application. Full article

In this article, energy optimization of the cooling system of IKEA Budaörs is carried out. The cooling system is served by a centrifugal water chiller and includes a large-volume cooling buffer tank. The facility operates the hydraulic system of the buffer storage tank only during the transitional period. The main goal is to reduce energy consumption by changing the operating strategy of the existing system. To test the operating strategies, the operation and the thermal load of the shopping center during the summer season had to be simulated to find the best operation strategy. A hybrid method (real data and calculated values) was used in the simulation. The three operating scenarios examined show that the annual energy consumption and the number of operating hours of the chiller can be reduced by using the buffer tank with the right strategy. In the examined scenarios, a 30% energy improvement was achieved. The possibility of using a buffer tank is significantly limited by the fact that the heat exchangers were sized for low forward water temperatures. By re-sizing the heat exchangers, the utilization of the buffer tank could be considerably improved in conditions close to peak heat load. Full article

In standard district cooling (DC) plants, central chillers produce cold energy for space cooling throughout the district network. In recent times, the integration of the vapor compression system, which includes the functionalities of vapor compression chillers (VCC), and thermal energy storage (TES) tanks in the DC setup, has gained more implementation across the globe. This is due to the possibility of load shifting by using the VCC to produce chilled water for charging the TES tanks during off peak periods. Since the environmental implications of various energy intensive systems are largely determined by the amount of material and energy consumed throughout their life cycle, it is critical to conduct a sustainability assessment of these systems in terms of environmental contributions, and suggest design options to reduce these impacts. A cradle to grave life cycle assessment (LCA) model is created in response to these issues and in order to meet the project’s objectives. The life cycle impact assessment (LCIA) results of the analysis reveal that the carbon footprint per 1 RTh of the produced chilled water is estimated at 0.72 kg CO₂ eq/RTh. The operation phase of the system’s life cycle accounted for the most impact, about 98%, with other life cycle phases having negligible contributions. In substantiating the study’s investigation, the environmental performance based on several design options were discussed and compared to the case study. Among the several scenarios considered, incorporating the Sweden mix technology provided the case study with the most significant environmental savings, of about 94%. Full article

Liquid entrainment in a flooded evaporator has an important impact on the performance and safety of a water-cooled centrifugal chiller. In this paper, two methods for measuring the liquid entrainment factor in the evaporator of a centrifugal chiller based on energy balance are proposed. Method 1 involves only the heat exchange capacity of the evaporator and Method 2 involves both evaporator and condenser. The applicable conditions of the methods are discussed. Experimental measurements on the flooded evaporator of a single-stage water-cooled centrifugal chiller with refrigerant R134a show that, for a system with good thermal balance, there is little difference in the entrainment factor values obtained by the two methods. Method 2 was found to have slightly higher measurement accuracy, compared to Method 1. The uncertainty propagation analysis shows that for Method 2, the inlet and outlet water temperatures of the evaporator and condenser, motor input power, motor efficiency, transmission power loss and compressor suction and discharge temperatures are important factors. The experimental results show that the variation of the evaporator entrainment factor with refrigerant charge amount is different for different cooling capacity. At 700 and 800 refrigeration ton (RT), the entrainment factor of the test evaporator increases with the increase of refrigerant charge and the growth rate gradually accelerates. For the chiller tested, when the entrainment factor reaches 0.89% and 1.02%, respectively, at 700 ton and 800 ton, the rapid increase of the entrainment factor leads to a significant decrease in the coefficient of performance (COP) during the charging process. Based on the analysis of the experimental results, it is recommended that the maximum entrainment factor for efficient operation of the centrifugal chiller should be controlled within 1%. Full article

The cooling coefficient of performance (COP_R) and energy efficiency ratio (EER) of refrigerant R-134a compressors (single- and double-compressors) with different refrigerant tonnage (200, 250, 300, 380, 500, and 700 RT) for centrifugal and Maglev centrifugal compressors change with different operating performance load percentages (10–100%), and constant-frequency and variable-frequency operation, resulting in performance differences. In particular, a water chiller can have a fixed cooling water inlet temperature of 32 °C and a variable cooling water inlet temperature between 18.33 °C and 32 °C. According to the actual test results, the commercial performance code program and parameter table of the water chiller were established. Based on the performance matching of different load chillers, the on-site load capacity was analyzed and the effective water chiller performance and model matching were determined as the best choice for the ton_R number of the deicing machine and unit matching, providing a reference for a future large water chiller that cannot be used on site for a single unit ton_R. To achieve energy-saving benefits, different types of compressors, different refrigeration ton_R operation, constant-frequency unit and variable-frequency unit alternate operation, and different operating performance load percentage operation can be allocated. Finally, the results show that, when the cooling water inlet temperature is fixed, the Maglev variable-frequency centrifugal compressor water chiller is better than the constant-frequency centrifugal water chiller, and also better than the variable-frequency centrifugal water chiller. The larger the freezing ton_R of the variable-frequency centrifugal water chiller, the smaller the difference between COP_R and EER. When the cooling water inlet temperature changes, the Maglev variable-frequency centrifugal water chiller is better than the constant-frequency centrifugal water chiller, and it is also better than the variable-frequency centrifugal water chiller. The larger the freezing ton_R of the variable-frequency centrifugal water chiller, the smaller the difference between COP_R and EER. Moreover, the operating performance of the constant-frequency centrifugal water chiller is between 60% and 90%, which can maintain relatively high COP_R and EER values. The operating performance of the variable-frequency centrifugal water chiller is between 40% and 70%, which can maintain relatively high COP_R and EER values. Compared with the constant-frequency and variable-frequency, the Maglev variable-frequency centrifugal water chiller can maintain higher COP_R and EER values when the operating performance is between 10% and 100%. When the operating performance is between 10% and 70%, it can maintain very high COP_R and EER values. When the water chiller is selected in the field, the energy-saving of COP_R and EER will be given priority. Therefore, the load capacity can be used to effectively manage the water chiller performance and model selection, so that the operation performance can reach the best percentage and energy saving can be achieved. Full article

District cooling (DC) systems have recently proven to be more economically and environmentally viable as compared to conventional cooling techniques. In most DC setups, electric centrifugal chillers (ECCs) are installed to provide chilled water (CW) to charge the thermal energy storage (TES) tank or for direct CW supply to the DC network. The operation of these ECC systems consumes most of the electrical power supplied to the entire DC plant; this therefore strengthens the need to conduct a comprehensive environmental assessment in order to quantify the indirect ecological impact resulting from the energy consumed in the ECC system operation. In order to achieve this, a case study was conducted of four ECC systems with a use-life of 25 years installed in a large DC plant in Malaysia. A gate-to-gate life cycle assessment (LCA) methodology was adopted to analyze the environmental performance of the system setup. The result of the study year reveals that April and June account for the highest and lowest environmental impact, respectively. The influence of climatic temperature conditions on the monthly cooling and environmental load distribution was also observed from the results. Finally, in substantiating the study’s investigation, environmental performance based on the composition of two different electricity fuel mixes is discussed and compared. The results revealed a drastic decrease in environmental load as the ratio of non-renewable energy sources decreased in the composition of the mix, thereby reducing the contribution of the overall environmental impact of the ECC systems’ use phase. Full article

The Gordon-Ng models are tools that have been used to estimate and evaluate the performance of various types of chillers for several years. A 550 TR centrifugal chiller plant facility was available to collect data from July and September 2018. The authors propose rearranging variables of the traditional (GNU) model based on average electric consumption and through a thermodynamic analysis comparable to the original model. Furthermore, assumptions are validated. Then, by estimation of the parameters of the new model using least square fitting with field training data and comparing to the GNU model and Braun model (based on consumption), it was shown that the proposed model provides a better prediction in order to evaluate consumption of a centrifugal chiller in regular operation, by improving the coefficient of variation (CV), CV = 3.24% and R² = 92.52% for a filtered sub-data. Through an algorithm built from steady-state cycle analysis, physical parameters (S_gen, Q_leak,eq, R) were estimated to compare with the same parameters obtained by regression to check the influence of the interception term in the model. It was found that without an interception term, the estimated parameters achieve relative errors (ER) below 20%. Additional comparison between external and internal power prediction is shown, with CV = 3.57 % and mean relative error (MRE) of 2.7%, achieving better accuracy than GNU and Braun model. Full article